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WD2 17.30–17.45 A Distributed Bragg Reflector Silicon Evanescent Laser Alexander W. Fanga, Brian R. Kocha,b, Richard Jonesb, Erica Lively a, Di Liang a, Ying-Hao Kuo a, and John E. Bowersa a University of California Santa Barbara, ECE Department, Santa Barbara, CA 93106, USA b Intel Corporation, 2200 Mission College Blvd, SC-12-326, Santa Clara, CA 95054, USA Email: [email protected] Abstract We report a distributed Bragg reflector silicon evanescent laser operating continuous wave at 1596nm. The lasing threshold and maximum output power are 65mA and 11mW, respectively and shows open eye-diagrams under direct modulation at 2.5 Gb/s. I. Introduction Recently, hybrid integration has received a lot of attention as a method to create electrically pumped laser sources on silicon. This new interest is due to developments in the transfer of thin crystalline III-V films to silicon [1,2] leading to the demonstration of micro-disk, and Fabry-Perot III-V membrane lasers coupled to silicon waveguides [3,4] and Fabry-Perot, and racetrack hybrid silicon evanescent lasers [5], These devices allow for the use of electrically pumped III-V gain regions while enabling scalable manufacturing through alignment free bonding that is absent in conventional gold bump bond die attach of III-V active devices. Hybrid silicon evanescent lasers (SEL) utilize both III-V regions and silicon waveguide regions within the device, allowing for processing in the silicon region to define the cavity along with lasing properties. Earlier this year, we demonstrated a distributed feedback (DFB) SEL that utilized surface corrugated gratings beneath the bonded III-V region to realize an electrically pumped single wavelength laser on silicon [6]. Here, we demonstrate a distributed Bragg reflector (DBR) SEL where passive gratings are placed on both sides of the active region in order to form a wavelength selective cavity. Figure 1: (top panel) DBR-SEL side-view topographical structure, (center panel) DBR-SEL top-view topographical structure, (bottom panel) Microscope image of DBR-SEL II. Device Structure The DBR-SEL is fabricated on the silicon evanescent waveguide platform as described in reference [6]. The silicon waveguide has a width, height, and rib etch depth of 2 μm, 0.7 μm, and 0.5 μm, respectively. This results in silicon and quantum well confinement factors of 66 % and 4.4 %. A 440 micron long silicon evanescent gain region and two 80 micron long tapers are placed inside the cavity. The tapers provide an adiabatic transition between the passive silicon regions and the silicon evanescent waveguide regions by varying the width of the upper III-V layers along the length of the taper, reducing the reflection and allowing for low loss coupling between these two regions. The device topography consists of a two passive Bragg reflector mirrors placed 600 microns apart to form an optical cavity. The back and front mirror lengths are 300 microns and 100 microns. The surface corrugated gratings are formed during silicon processing prior to wafer bonding through ebeam lithography. They have an etch depth and duty cycle of 25 nm and 75 %, respectively, with an upper cladding of SU-8 leading to a grating κ of 80 cm-1. The power reflectivity of the gratings can be calculated using the following expression [7]: ܴ ൌ ݄݊ܽݐଶ ሺߢܮሻ resulting in power reflectivities of 97% and 44% for the back and front mirrors, respectively. 978-1-4244-1768-1/08/$25.00©2008 IEEE III. Experimental Results The laser output power is measured with an integrating sphere at the front mirror of the laser. The front mirror L-I is shown in Figure 2. The device has a lasing threshold of 65 mA, a maximum device output power, of 11 mW, and a differential efficiency of 15%. The laser operates up to 45 °C. The kinks in the LI are from mode hopping and will be discussed later. The device has lasing turn-on voltage of 2.4 V and series resistance of 11.5 ohm. 58 the current sweet. In intermediate regimes where a dip occurs in the L-I, the lower adjacent mode has not moved into the spectral gain-reflectivity peak and other adjacent modes lase. Figure 2: DBR-SEL L-I curve for various temperatures measured out of the front mirror. The lasing spectrum is shown in Figure 3 with a lasing peak at 1597.5 nm when driven at 200 mA. The device has a free spectral range (FSR) of 0.47 nm, which corresponds to a group index of 3.86 based on the sum of the physical cavity length and mirror penetration depths of 61 and 42 microns. The side mode suppression ratio is 50 dB. Figure 4: L-I curve and spectrum versus current at a stage temperature of 18 °C. We measured the modulation characteristics of the device by using a bias-T to drive the laser simultaneously with a DC current and an RF signal while measuring the electro-optic (EO) response on a photodetector. Figure 5 shows the overall photodetected EO response of the laser with all connected components under small signal modulation (-10 dBm). S11 measurements indicate that the electrical contact geometry is not limiting the performance of the device, so reflected power has not been factored out of these curves. In addition, a 2 pF device capacitance was extracted from the S11 measurement, resulting in an RC limited bandwidth of 7 GHz. As expected, when the DC bias current on the laser increases the resonance frequency increases. Under higher modulation powers the resonance peak is significantly damped. The 3 dB electrical bandwidth at 105 mA is ~2.5 GHz. Figure 3: Optical spectrum of the DBR-SEL driven at 200 mA Figure 4 shows the lasing spectrum as a function of drive current along with the corresponding L-I curve. It can be seen that as the device heats with larger current injection, the lasing mode moves to longer wavelengths due to the thermo-optic effect in the cavity. When the mode moves far enough from the reflection peak of the mirrors and the gain can no longer support this lasing mode, a longitudinal mode hop to a lower mode occurs. It can be seen from the L-I that the transition between these two adjacent modes is not continuous through Figure 5- Photodetected frequency response of the DFBSEL for 3 different bias currents with a stage temperature of 18 °C We directly modulated the laser biased at 105 mA DC current with a 2.5 Gb/s, 231-1 PRBS electrical signal having 59 insulator waveguide circuit” Optics Express, Vol. 15, Issue 11, pp. 6744-6749 [4] G. Roelkens et al., “Laser emission and photodetection in an InP/InGaAsP layer integrated on and coupled to a silicon-oninsulator waveguide circuit,” Opt. Express 14, 8154-8159 (2006). [5] A. W. Fang, et al., “Electrically pumped hybrid AlGaInAs-silicon evanescent laser,” Opt. Express 14, 9203-9210 (2006). [6] A. W. Fang, et al., “A distributed feedback silicon evanescent laser,” Opt. Express 16, 4413-4419 (2008). [7] Michael Bass, Handbook of Optics IV, McGraw-Hill (2001), page 9.4 20 mW of RF power. The resulting eye diagram is shown in Figure 6. The extinction ratio is 8.7 dB. Although the modulation bandwidth increases at higher DC currents, the extinction ratio decreases unless larger RF modulation powers are used. For example an open eye diagram at 4 Gb/s can be obtained, but it has an ER closer to 6 dB. Improving the laser design to decrease the threshold current and increase the differential gain is expected to significantly improve the modulation bandwidth in future devices. Figure 6- Eye diagram of a 2.5 Gb/s directly modulated DBR-SEL. IV. Conclusions We report a distributed Bragg reflector laser on the silicon evanescent platform operating in the 1596 nm regime with a side mode suppression ratio of 50 dB. The laser operates continuous wave with a lasing threshold of 65 mA and maximum output power of 11 mW at 15 C. The laser showed open eye diagrams with extinction ratios of 8.7 dB and 6 dB for data rates of 2.5 Gb/s and 4 Gb/s, respectively. This demonstration paves the way for the use of directly modulated lasers on silicon integrated with silicon interleavers or arrayed waveguide gratings in order to create low cost, silicon wavelength division multiplexed transmitters. V. Acknowledgements The authors would like to thank J. Shah, M. Haney, D. Blumenthal, L. Coldren, M. Paniccia, H. Park, and H.-W. Chen for insightful discussions. This work was supported by a grant from Intel Corp. and from DARPA/MTO DODN program and ARL under award number W911NF-05-10175 and W911NF-04-9-0001. VI. References [1] D. Pasquariello, et al., “Plasma-Assisted InP-to-Si Low Temperature Wafer Bonding,” IEEE J. Sel. Topics Quantum Electron. 8, 118, (2002). [2] Q. Tong et al., “Low temperature InP/Si wafer bonding,” Appl. Phys. Lett. 84, 732, (2004). [3] J. Van Campenhout, et al., “Electrically pumped InP-based microdisk lasers integrated with a nanophotonic silicon-on 60